Wireless Local Area Networks (WLANs) have become an important part of everyday life. Users (e.g., travelers, office workers, factory workers, etc.) can often gain access to communication systems (e.g., voice, e-mail, Internet, etc.) using a wireless device from any of a variety of locations (e.g., airport, office, factory, etc.).
Wireless devices used in conjunction with a WLAN typically operate under an appropriate IEEE standard (e.g., IEEE 802.11a) over a relatively short distance (e.g., 300 feet). In order to obtain continuous coverage, WLAN base stations must be placed at sufficiently short intervals as to provide overlapping service.
Even in the presence of overlapping WLANs, service may not be reliable for a number of reasons (e.g., Rayleigh fading). In order to overcome these difficulties, at least some WLAN devices rely upon the use of two or more antennas using a process called selection diversity. Under selection diversity, a controller within the WLAN device measures the signal from each of the antennas and selects the antenna providing the strongest signal.
Selection diversity is used, as opposed to more optimal antenna combining techniques, such as maximal ratio combining, because it has lower cost and less complexity. Selection diversity can be implemented with a single radio frequency front-end (e.g., using filtering and downconversion), a single complex analog-to-digital converter, and a single baseband demodulator that are shared, in a time-multiplexed fashion, between the two antennas. Conversely, maximal-ratio combining of the antennas and other similar techniques require a radio frequency front-end, a complex analog-to-digital converter, and a demodulator per antenna.
While selection diversity is effective for slow moving devices, it fails to provide reliable service where the user is moving (e.g., riding in an automobile), in that the best antenna (providing the strongest signal) can change from frame to frame, or even within the time span of a single frame. Accordingly, a need exists for a method of ameliorating the effects of movement that is compatible with the existing standards.